Sort of motor not allowed by directors (8)
I believe the answer is:
outboard
'sort of motor' is the definition.
(I know that outboard is a type of engine)
'not allowed by directors' is the wordplay.
'not allowed' becomes 'out' (I've seen this before).
'by' says to put letters next to each other.
'directors' becomes 'board' (board of directors).
'out'+'board'='OUTBOARD'
(Other definitions for outboard that I've seen before include "Engine attached to boat" , "Small boat's motor" , "Type of boat engine" , "Sort of motor" , "Engine external to the boat" .)